This invention relates to a dual motor, single axis position and velocity servo system for a patient-supporting table that is used to position a patient in an X-ray beam in computed axial tomography and computed projection radiography apparatus, for example.
As is known, in one type of computed axial tomography apparatus an X-ray source is mounted on a scanner base that is journaled for rotation about a nominally horizontal axis on a tiltable gantry. A multiple cell X-ray detector is mounted on the scanner base on the opposite side of the axis from the X-ray source. The X-ray beam emanating from the source is collimated into a fan-shaped configuration that spreads over the circumferential length of the detector and is thin in the direction to which the rotational axis of the scanner base is perpendicular. The patient who is to be examined is customarily supported on an X-ray transmissive table top or cradle in coincidence with the rotational axis of the scanner. The cradle must be supported from a floor-mounted base in a fashion that allows it to be moved in what is called the axial or longitudinal direction relative to the base so that the X-ray beam may be caused to penetrate successive layers of the body. As is well known, the computed axial tomography process requires orbiting the X-ray source and detector jointly about the patient so that the detector will be able to produce analog signals representative of X-ray beam attenuation by the patient for a multiplicity of X-ray source and detector positions that are attained in a 360.degree. orbit in one direction or the other. As is well known, the signals representative of beam attenuation are variously processed with a computer system that yields digital data representative of the intensity of the picture elements that comprise an image in axial perspective of the body layer that has been scanned. The picture element data is converted to analog video signals and is used to display the image on the display screen of a video monitor.
The gantry allows the scanner to be tilted so that the fan-shaped X-ray beam, instead of being projected perfectly vertically, is projected at an angle relative to the horizontal axis to permit imaging angular rather than vertical body layers. Pre-existing computed axial tomography apparatus typically provided for tilting the plane of the fan-shaped beam through an angular range of about 10.degree. from either side of vertical. Since the gantry is a large upright structure, when tilted through a small angle such as 10.degree., its bottom swings out toward the base on which the axially movable patient-supporting cradle is mounted but the base can be set far enough away from the gantry to avoid having the latter strike the base. This is so because the X-ray transmissive cradle supported the patient in cantilever fashion from the base. However, in a more advanced computed axial tomography apparatus design, tilting of the gantry about 20.degree. or more from either side of vertical has been provided for. Thus, the base that supports the cradle must be set farther away from the gantry to avoid interference by the base when the gantry is tilted through the larger angular range. Hence, in the new design it became necessary to increase the distance through which the patient is translated axially relative to the base. It would be possible to lengthen the cradle so a major part of the patient could be advanced into the X-ray beam but, since the patient would be supported in cantilever fashion, intolerable deflection of the cradle would result. The alternative that was adopted is to make the patient-supporting table assembly in two sections comprised of an intermediate support or carriage that moves relative to the base and a cradle carriage mounted on the intermediate support for moving axially relative to it. The patient cradle itself is an X-ray transmissive member that extends in cantilever fashion from the cradle carriage.
In connection with the computed projection radiography method, using the apparatus described briefly above, the X-ray source and detector are held in a fixed position rather than being orbited to perform a scan as in the computed axial tomography method. In this method, the patient must be advanced through the fan-shaped X-ray beam at a very constant velocity for undergoing a line-by-line scan with a fan beam that is about 1.5 mm thick, for example. As the patient is being advanced, the X-ray detector cells yield analog signals corresponding to X-ray attenuation at closely successive positions of the patient on a line-by-line basis and the resulting attenuation data is stored until the length of the body which is of interest has been scanned. A computer then uses the attenuation data to produce digital data representative of the intensities of the picture elements for all scan lines and these signals are used to drive a video monitor which displays a visual image corresponding to the projected X-ray image. In effect, the computed projection radiography method yields a visual X-ray image that is comparable to the image obtained with ordinary radiographic film but with greather contrast than is obtainable with film because the dynamic range of the X-ray detector is usually greater than that of film.
When a sectionalized patient table comprised of an intermediate support and a cradle functioning in what may be called telescopic fashion is used, the support and cradle must be moved in sequence to move the patient toward or away from the X-ray beam. The preferred thing to do is drive the intermediate support and cradle with individual motors. However, this creates the problem of having one motor come to a stop and having the other motor take over without any change in the translational velocity of the patient. A velocity change would result in an X-ray image that has severe artifacts. The invention described herein solves the problem without degradation of position or velocity accuracy and without the need to use more than one decoder.